Some prior filming methods utilized solvent-based filming lacquers which were deposited onto a wet or prewetted phosphor layer to produce a thin, continuous film on which the subsequently evaporated metal layer is condensed. The solvent-based lacquers were deposited by a spray-filming process, known in the art, which is very sensitive to variations in spraying parameters and requires special safety and environmental controls, because of the volatile nature of the solvent base. While such solvent-based films provide excellent results and permit a combined panel bake and frit seal of the panel to the CRT funnel, current practice is to use a water-based emulsion film.
U.S. Pat No. 3,067,055, issued to T. A. Saulnier, Jr., on Dec. 4, 1962, describes a process for coating the screen with such an aqueous emulsion. The emulsion contains a copolymer of an alkyl methacrylate and methacrylic acid. The panel bearing the screen is heated as the aqueous emulsion is coated thereon, to produce a dry, volatilizable film. A layer of metal, such as aluminum, is deposited on the surface of the film, and then, the film is volatilized. The purpose of the film is to bridge the irregular surface of the phosphor particles which make up the screen, and to provide a taut, suspended substrate onto which the evaporated metal is condensed to form a mirror-like surface, to reflect light from the screen. Although the film is pierced by many of the protruding phosphor particles on which it is suspended, these particles serve to plug or obstruct the holes in the film and, thus, prevent significant metal penetration to the underlying screen. Vacuum deposition of the metallic layer leaves it suspended in an exact replica (including the holes) of the film's top surface. The screen is then "baked-out" at an elevated temperature of about 410.degree. to 440.degree. C. to remove the film and the organic binders of the phosphors. The gaseous products formed during "bake-out" escape through the holes in the metal layer. A drawback of the process, described in U.S. Pat. No. 3,067,055 is that the single coating of the disclosed filming material provides a system which is difficult to control for light output, and which requires a slow prebake of the screen, in order to remove substantially all of the organic materials.
U.S. Pat. No. 4,217,015, issued to S. Matsumoto et al., on Aug. 12, 1980, describes a CRT-making process in which a thicker film, with a higher acrylic resin content (34 to 40% by weight), is used to increase the brightness of a tube having a "no-mill" (i.e., non-ball milled) phosphor screen. Frit sealing of the panel to the funnel is performed simultaneously with the bake-out of the screen; however, a hollow pipe is disposed within the tubular neck portion of the funnel to provide sufficient air, within the sealed envelope, to volatilize the organic binders in the phosphor screen and to remove the acrylic resin film. The pipe tends to permit dust particles to be drawn into the tube envelope from the sealing furnace and also scratches the neck of the funnel, resulting in neck cracks during stem sealing. Additionally, modification of the sealing furnace is required to accommodate the pipes, one of which fits into the neck of each tube.
U.S. Pat. No. 4,350,514, issued to Akiyama et al., on Sep. 21, 1982, discloses a method of simultaneously baking-out the organic constituents of the phosphor screen and the film, and sealing the panel to the funnel. Oxygen-rich air is introduced into a sealing furnace in the zones of the furnace where the temperature increases from about 300.degree. C. to portions of the furnace where the temperature reaches about 450.degree. C. The oxygen-rich air is sufficient to volatilize the organics of the screen and the film.
Each of the aforementioned processes has one or more drawbacks including safety and environmental considerations, insufficient screen brightness caused by filming difficulties, special equipment required to provide sufficient oxygen to decompose thick film layers, tube scrap caused by drawing dirt into the tube and sealing scrap resulting from scratches in the neck of the funnel. The present invention is directed to a manufacturing process utilizing an aqueous emulsion that does not require a dedicated panel bake prior to frit seal, but which permits a simultaneous screen bake and frit seal, without the need for introducing additional oxygen into the tube. Tubes produced by this process have a screen brightness at least comparable to that achieved using a dedicated panel bake.